The EU's chief Brexit negotiator Michel Barnier said the plan "raises more questions than it answers."

He said the time-limited nature of the plan "doesn't work for us."

LONDON — The EU's chief Brexit negotiator has cast serious doubts over the UK's "backstop" plan to avoid a hard Irish border by keeping the UK in the customs union, but denied rejecting it fully.

Michel Barnier said in Brussels on Friday that the UK's newly-published "backstop" proposals — an emergency measure designed to avoid the emergence of a hard border in Ireland — would not work if they sought to keep the whole of the UK in the customs union.

"Let me be very clear: Our backstop cannot be extended to the whole UK," he said. "Why? Because it has been designed for the specific situation of Northern Ireland."

He suggested the EU's own proposal to keep only Northern Ireland within the EU customs union was workable but said: "What is feasible with a territory the size of Northern Ireland is not necessarily feasible with the whole UK."

He said he would continue to examine the backstop paper "objectively" but stressed it would be rejected if it compromised the integrity of the single market and customs union.

He also said that the references to a "time limited" backstop arrangement — which were inserted at the last minute on Thursday after Brexit Secretary David Davis threatened to resign — would be rejected by the EU.

"The time-limited terminology doesn't work for us," he said. "The temporary backstop is not in line with what we want or what Ireland or Northern Ireland need."

Prime Minister Theresa May on Thursday published a plan for a backstop which would take effect if an EU-UK deal was not in place by 2021.

It is designed to avoid the emergence of a border between Northern Ireland and Ireland, and the EU will only sign a transition deal with the UK when its details have been agreed.

Downing Street issued a response to Barnier's comments on Friday afternoon. It said the UK government would not accept an Ireland-only backstop, which is what the EU has proposed.

A government spokesperson said: "The Prime Minster has been clear that we will never accept a customs border between Northern Ireland and the rest of the United Kingdom.

"We are also committed to maintaining the integrity of our own internal market. That position will not change. The Commission's proposals did not achieve this, which is why we have put forward our own backstop solutions for customs.

"All parties must recall their commitment in the Joint Report to protect the Belfast Agreement in all its parts.

"Michel Barnier has confirmed today that discussions will now continue on our proposal."

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